1. Field of the Invention
The present invention relates to a catadioptric optical system having a reflection surface and a refraction surface, and more particularly to an image forming optical system for reduction projection.
2. Related Background Art
Various optical systems for projecting and exposing a mask pattern onto a photoresist on a wafer to manufacture an integrated circuit such as an LSI have been proposed. It is known that an aberration is well corrected at a relatively large aperture in a Dyson type catadioptric optical system. However, since a focusing magnification in the Dyson type catadioptric optical system is unity, there is a limit in the transfer of a fine pattern.
As an optical system having a reduction factor suitable for the manufacture of a semiconductor device having a finer pattern, an optical system which is a modification of the Dyson type catadioptric optical system is disclosed U.S. Pat. No. 4,747,678 and U.S. Pat. No. 4,953,960.
The optical system disclosed in U.S. Pat. No. 4,747,678 which permits the reduction projection allows the reduction focusing of a ring-shaped view field. Basically, it comprises three concave mirrors and a convex mirror and is used as a light exposure device for microlithography. Accordingly, it uses a very complex optical construction of a combination of a number of lenses.
The reduction projection optical system disclosed in U.S. Pat. No. 4,953,960 comprises a combination of a concave mirror and a lens system. Since the reduction is attained by one time of focusing, the aberration is large and a combination of a number of lenses is needed to correct the aberration. Since it is essential to provide a large scale beam splitter such as a half mirror in a light path, a loss of light intensity is more than 75% and the increase of exposure time due to the loss of light intensity causes a fatal shortcoming of the reduction of throughput in the manufacturing process of the semiconductor device. Further, a stray light which causes flare and ununiformity in illumination occurs in the large beam splitter in the light path, and it is difficult to manufacture a large semitransparent plane.